This invention relates to a process for producing an emulsion for a coating material to be applied to paper, a plastic film or an inorganic substrate and to an emulsion produced by the above production process. More particularly, it relates to a process for producing a cationic emulsion which is stable to alkalis, has a small average particle size and can be cured at ordinary temperature, and consequently gives good water resistance and good permeability to a strongly alkaline substrate such as an inorganic substrate or the like when coated and which further can provide such a function as a coating material that when the emulsion is coated on paper or a film, its adherability thereto and water resistance are developed, and also relates to a cationic emulsion produced by the production process.
As materials to be coated on a substrate for forming a coating film, solvent type resin compositions have heretofore been used. However, the above solvent type resin compositions have problems with safety and regulation for volatile organic solvent, and as substitutes therefor, water-soluble resins and emulsion resin compositions have been used. As an example thereof, proposed is a cationic microemulsion which comprises fine particles having particle diameters of not more than 0.1 microns and which has a cross-linkability at ordinary temperature and forms a dense coating film having a gloss close to that in the case of the solvent type resin composition. However, with the above microemulsion, a large amount of an emulsifier must be used for making the particle diameter 0.1 microns or less, and consequently, the above cationic microemulsion is disadvantageous in that its water resistance becomes remarkably inferior and in other respects.
Recently, there has been proposed a cationic emulsion having a core/shell structure formed by effecting emulsion-polymerization using a water soluble resin and also an epoxy group without any emulsifier (for example, JP-A-Hei 6(1994)-1,680 and JP-A-Hei 6(1994)-1,928). However, even in these cases, how to use the water soluble resin as the shell component is a great factor, the curing rate is not sufficient and a lot of time is required until water resistance is developed. In addition, when the proportion of the water-soluble resin is increased for ensuring the stability, the water resistance is remarkably deteriorated, and hence, the development of those which can be used without anxiety is now awaited.
An object of this invention is to provide a cationic emulsion which has a good adherability to paper, plastic film or inorganic substrate and is excellent in water resistance and long-term stability.
Another object of this invention is to provide a process for producing the above cationic emulsion.
A still another object of this invention is to provide an article coated with the above cationic emulsion.
Other objects and advantages of this invention will become apparent from the following description.
The present inventors have continued research for the establishment of a process for producing a cationic emulsion which is endowed with both water resistance and adherability to various substrates, the emulsion characteristics of which are excellent in both property and physical property and stable and for the development of said cationic emulsion and have consequently grasped that said emulsion can be produced by adopting the following technique.
It is needless to say that the emulsion stable to alkalis is cationic; however, the present inventors have found that the object can be successfully achieved only when the following (1) to (3) are combined on the above premise, whereby this invention has been accomplished:
(1) The emulsion is required to be of fine particles in order to increase the permeability and for this purpose, it is necessary to increase the amount of a water-soluble resin used as the shell component; however, when this amount is increased, the water resistance is deteriorated, so that it is necessary to bond the core portion to the shell portion.
(2) In order to ensure the adhesiveness, it is effective to introduce an epoxy group. In this case, the object is substantially achieved by selectively subjecting the chlorine atom of epichlorohydrin as used to reaction at a temperature not higher than ordinary temperature; however, the reaction requires a long period of time and consequently the production man-hour becomes large. This is disadvantageous. Moreover, in this technique, the epoxy group is alive, and hence, the storage stability at high temperatures cannot be secured. This is not desirable. Accordingly, it has been examined how much the stability is adversely affected by subjecting epichlorohydrin to reaction at a high temperature to allow the epoxy group to react selectively and thereafter spontaneously reacting the chlorine atom whose reaction rate is low, whereby it has been ascertained that both the ensuring of stability and the ensuring of adhesiveness can stand together without any trouble.
(3) If necessary, an amphoteric water-soluble resin having both cation and anion can be used as the shell component in order to obtain excellent stability, adhesiveness and water resistance.
According to this invention, there is provided a process for producing a cationic emulsion which is stable to alkalis and has an average particle diameter of 0.01 to 0.1 micron, in which using, as an emulsifier, a water-soluble polymer A2 obtained by subjecting a polymer A1 having a weight average molecular weight of 5,000 to 100,000 obtained by polymerizing an amino group-containing, polymerizable compound and a compound copolymerizable therewith in the presence of a polymerization initiator, to neutralization reaction with an organic or inorganic acid in a proportion of 0.5 to 1.5 equivalents per equivalent of the amino group brought about by the above amino group-containing, polymerizable compound, an epoxy group-containing, polymerizable compound C represented by the general formula (I): 
wherein R1 represents H or a methyl group and R represents a straight chain or branched chain alkyl group having 1 to 10 carbon atoms and a compound copolymerizable with the compound C are subjected in combination in a proportion of 0.01 to 0.5 equivalent per equivalent of the above amino group as elements to constitute an oil-soluble core portion B1 to emulation polymerization in the presence of a redox catalyst system in water as a medium to obtain an emulsion D having a weight average molecular weight of at least 100,000 and having a core/shell structure in which the core portion and the shell portion are integrated by the bonding of the above amino group to the above epoxy group; this emulsion D is then reacted with epichlorohydrin in an amount of at least 0.5 equivalent per equivalent of the amino group at 20 to 100xc2x0 C. to selectively add the epoxy group to the amino group; and thereafter, the chlorine atom resulting from the epichlorohydrin is reacted with the amino group.
According to this invention, there is further provided a cationic emulsion of an average particle diameter of 0.01 to 0.1 micron stable to alkalis produced by the above-mentioned production process.
According to this invention, there is still further provided an article coated with the above cationic emulsion.